Decided to purchase the GIVI SR683 frame and pay the bucks for a larger Monokey case. What a great piece of gear. Solid as a rock, easy fit, and doesn't look too out of place. For those who have been stuffing about with 'manufactured' racks and fittings like me, this is an excellent option that is thoroughly recommended if you need a solid rack for pillions or large cases. Pricey here in Aus, but seems to be money well-spent. It comes standard with a GIVI Monokey base plate.
I know that the big topcases don't suit everyone, but I find it useful as a 'suitcase' that leaves the side cases for spares, tools, wets, extra gloves, jacket liner, first aid kit and so on. I always travel with the side cases, but now just leave the topcase off unless a trip is on.
If you have been contemplating a big topcase, this is definitely the way to go, especially as the GIVI gear seems to be a good bit cheaper Stateside than here (even allowing for a good exchange rate).
EDIT - Don't confuse this SR683 with an earlier version that only had two uprights and didn't bolt to the rail 'wings'. It just used the fwd and rear rail mounts. This one uses the rear rail mounts and 'wings'. The earlier version also needed the charcoal cannister to be removed.